The invention relates to oxygen sensors and particularly to sensors for use in industrial air/fuel control systems.
Sensors to be included as parts of industrial air/fuel control systems must meet requirements peculiar to industrial air/fuel control. In general, air/fuel ratio control in an industrial system is done in the excess air regime. This is done to insure that complete combustion of the fuel occurs and that no visible smoke stack emissions are produced. A second beneficial reason for controlling in this region has been a demonstrated improvement in combustion efficiency and resultant lowered fuel consumption.
In order that a sensor meet these requirements and still be commercially feasible, sensor performance must meet a number of criteria not normally significant for many other sensor applications. For example, sensor-to-sensor reproducibility is important. Without this demonstrated reproducibility, control system recalibration would be required at the time of each sensor replacement. Stability of the sensor signal over long term usage is also an extremely important feature, so that frequent system recalibration is not necessary. Another feature which is important in an industrial application is simplicity of installation of the sensor into the gaseous environment which is being measured. Finally, low sensor cost is important, particularly when replacement of sensors may be necessary.
The application of oxygen measuring sensors as either simple monitors of oxygen content or as sensors in control systems has received considerable interest in the patent literature. The earliest commercial application of oxygen sensing devices was as integral components in systems designed for measuring dissolved oxygen in molten metal baths. This area is still of considerable commercial importance as indicated by a number of recent patents in this area including U.S. Pat. Nos. 3,359,188 by Fischer, 3,378,478 by Kolodney, 3,403,090 by Tajiri and 3,791,954 by Noda. The devices covered in these patents are all similar in that they are solid electrolytic sensors which are immersed in a liquid metal and which generate an electrical signal which is a function of the oxygen content of the metal. The first three devices incorporate closed ended tubes of the electrolytic material while the fourth sensor uses a disc of solid electrolyte sealed into a retaining tube.
Most of the sensing devices utilized for measuring the oxygen content in molten metals are marketed as destructable sensors, in that they react with the molten metal being measured and are consumed in the course of the measurement. In U.S. Pat. No. 3,297,551 by Alcock a liquid metal sensor is presented which is used in the liquid metal heat exchanger of a nuclear reactor on a long term continuing basis.
Other sensors have been designed for measuring oxygen content specifically in gaseous mixtures rather than in liquid metals. For example, U.S. Pat. No. 3,974,054 by Poolman covers a disc electrolyte bonded to an aluminum oxide tube which is used for determining oxygen concentrations in gaseous mixtures. Another gas sensor is presented in U.S. Pat. No. 3,989,614 by Tien which specifies a sensor composed of a tubular solid electrolyte. In addition to the above, a number of oxygen sensors and sensing systems are commercially available, all of which use solid electrolytic sensing of oxygen.
Recently, there has been a considerable amount of interest in the measurement of the components which make up automotive exhaust gases. A number of devices have been patented for the measurement of the stoichiometric air/fuel ratio going into internal combustion engines. These are proposed to be used with control systems to hold engine operating conditions at the stoichiometric value. Proposed devices include both those that work as electrical resistance sensing elements as well as electrolytic sensors.
Typical resistance sensors have been covered in U.S. Pat. Nos. 3,911,386 by Beaudoin; 3,936,794 by Beaudoin and 3,959,765 by Stewart. These devices sense a change in the electrical resistance of ceramic elements which show a change in electrical resistance that can be correlated with ambient oxygen pressure.
Of greater interest, however, are those automotive oxygen sensors which operate on electrolytic principles. A number of patents have been issued in this area on sensors which have the electrolyte in the form of tubes or discs.
Tube sensor patents include British Pat. No. 1,385,464 by Bosch and U.S. Pat. Nos. 3,841,987 by Friese; 3,935,089 by Togawa; 3,960,693 by Weyl, and 3,978,006 by Topp. All of these patents specify automotive oxygen sensors where the solid electrolyte is in the form of an open-ended tube, the open end being in communication with the ambient air environment.
The disadvantages inherent in such tubular electrolytic sensors have led to the development of simplified automotive oxygen sensors incorporating electrolyte discs rather than tubes. The disadvantages of the tubular sensors include complexity and the resultant cost in producing the electrolyte tube, and the expense and difficulty involved in applying large amounts of costly electrode materials over the surface of the tube.
The development of sensors incorporating very simple electrolytic discs has circumvented these problem areas. Examples of such automotive oxygen sensors incorporating discs include U.S. Pat. Nos. 3,819,500 by VanEsdonk; 3,909,385 by Spielberg; 3,768,259 by Carnahan, and 3,940,327 by Wagner.
There are a number of disadvantages in applying either currently available industrial or automotive sensors to feedback control systems designed for use in industrial combustion control. Those sensors specifically designed for industrial use are generally of a large size are costly to replace, operate at high temperatures which limits useful life, or are designed to be used only once. In addition, those industrial sensors constructed with tubular electrolytes suffer the same disadvantage as tubular automotive sensors, that is high electrolyte cost and the necessity for relatively large electrodes made of expensive materials. Those industrial sensors commercially available generally operate at about 1500.degree. F. Continuous operation at this temperature causes deterioration of the electrode which seriously affects performance as well as thermal fatigue in the electrolyte when it is subjected to heating cycles.
Likewise, the application of sensors designed for automotive use to industrial air/fuel control has not generally been successful. Sensor-to-sensor reproducibility of sufficient accuracy has not been demonstrated with this type of sensor to be useful in controlling a specific oxygen content far from the stoichiometric value. In general, these automotive sensors have been designed as indicators only of the stoichiometric air/fuel ratio, rather than sensors which measure actual oxygen content quantitatively. They have also been designed to survive in the severe automotive environment. This design is not necessary for industrial air/fuel applications and results in a considerable unnecessary increase in cost and design complexity.
One problem which all of the automotive sensors seem to share is the introduction of undesirable electrical signals due to dissimilar material junctions. While this is not a problem in sensors used for stoichiometric air/fuel ratio definition, such signals can have extremely deleterious effects on sensors which are being used to quantitatively measure oxygen content, such as is necessary in off-stoichiometric air/fuel control in industrial applications.